This application pertains to the art of data transmission and more particularly data transmission for a controller based network. The invention is particularly applicable to a controller based network where data representing the status of events and devices is collected and periodically transmitted on to and from the network during scheduled time periods. The transmitted data is used in the generation of control commands for the monitored events and devices. However, it will be appreciated that the invention has broader applications for use in a variety of control systems transmitting data on a scheduled periodic basis.
Controller based networks employ sensors to monitor a wide range of events and devices and their associated physical characteristics including changes in pressure, flow, strain, temperature, time and state changes, among others. The data gathered by the sensors is placed onto the controller based network and provided to the controller, which analyzes the data and generates an appropriate output. The output can cause switches and valves to open and/or close, turn motors and pumps on and/or off, transmit additional data, or cause a temperature to increase and/or decrease, among other numerous and varied actions. By considering the manner in which data acquired by a sensor is transmitted onto a controller based network, existing networks can be divided into categories. A first of which is known as a Carrier Sense Multiple Access (CSMA) network. When data for this network is acquired, there is an immediate attempt to place the data onto the network, with a number of nodes attempting to access the network at the same time raising the possibility of data collision. If data acquired by a sensor cannot be immediately placed on the network, it is queued in a buffer while attempts to gain access are made. In an attempt to avoid these collisions, complex algorithms are employed which prioritize data and judge competing contentions. Therefore, a CSMA network is a "free-for-all" type system where it is not possible to predict when data from a particular node will gain access to the network.
Another category of network, based on the manner of data transmission, and to which the subject invention is directed, is a scheduled controller based network where data access to the network is performed at periodic scheduled times. In this type of network data is transmitted in a predetermined order during a network update interval (NUI). Data from a node follows data from a preceding node such that there is no attempt to access the network prior to conclusion of transmission of data from a proceeding scheduled node. By this arrangement, data collisions occurring during contention for access to the network are avoided. Though access is controlled, the network update time (NUT) for an individual node can vary. Particularly, the transmission time of data from a specific node will vary within set parameters, however, the order in which data from the nodes is transmitted is accomplished according to a preset schedule.
Existing networks of the second category have a characteristic of being conservative in determining when data is collected for a scheduled transmission to the network. This conservative approach is beneficial since attempting to collect data at too late a point in a network update interval (NUI) can result in there not being sufficient time to complete data collection before data is to be transmitted, requiring the data to wait for a next NUI. Thus, data may not be transmitted at all, or older than desired data may be transmitted, this also creates data jitter by disrupting the periodic deterministic nature of the data.
FIG. 1 illustrates a block diagram of a scheduled controller based network A as discussed above. Sensors 10a-10n acquire data that is input to adapter 12, which is an input/output device. An adapter of the type shown in FIG. 1 has the capability of concentrating a plurality of inputs and outputs to and from sensors 10a-10n. Adapter 12 may be one of Allen Bradley Corporation's 1771 family of adapters used in Allen Bradley Corporation's controller based networks such as the ControlNet network. ControlNet is a trademark of Allen Bradley Corporation. It is to be appreciated that while adapter 12 is used in this embodiment, other input/output devices which acquire sensed data may also be used.
Adapter 12 transmits the collected data onto network A. Controller 14, which operates program 16, uses the received data to determine the status of events or devices which are controlled by network A. Based on the received data, controller 14 transmits a command to adapter 18, which then provides the command to actuators 20a-20n used to adjust the operation of the events or devices controlled by network A.
As an example of the operation of network A, actuation of a switch (not shown) is sensed by sensor 10a. This data is input into a digital input card (not shown), and is transmitted onto network A through adapter 12. The data is passed to controller 14 which uses this information to generate a command to turn off a pump (not shown) by actuator 20a.
Controller 14 may be one of Allen Bradley Corporation's PLC-5/C series of controllers, however, it is to be appreciated that other controllers may also be used.
FIG. 2 illustrates the flow of data within network A of FIG. 1, including the scheduled periodic transmission of data and data collection. Two constant ten (10) millisecond network update intervals (NUIs) 30, 31 are shown in the figure. At the beginning of the first network update interval (NUI) 30 a tone 32 indicates the start of a periodic interval for communication (PIC). Following tone 32, transmission of data begins, with nodes 0-4 34.sub.0-4 scheduled for transmission. Nodes 0-4 34.sub.0-4 correspond to the devices, etc. monitored by sensors 10a-10n. The node transmission time during the first network update interval 30 is slightly longer than 3 milliseconds. Upon completion of data transmission by the fourth node 34.sub.4, an interrupt 37 is issued by controller 14 of FIG. 1. Interrupt 37 also doubles as a data collection start (DCS) signal. Therefore, issuing of interrupt 37 signifies not only that the scheduled data transmission of nodes 0-4 34.sub.0-4 has been completed, but also that collection of data for the next scheduled transmission during network update interval 31 is to begin. Interrupt 37 may be considered to be a fixed signal in that controller 14 is configured to issue interrupt 37 substantially immediately following the end of data transmission.
Thereafter, when tone 32 is again issued, transmission of data from nodes 0-4 34.sub.0-4 again takes place and the same procedure is repeated upon the issuance of interrupt, data collection start (DCS) signal, 37.
As illustrated by FIG. 2, while there may be a variability in the data transmission time such as from slightly more than 3 milliseconds during the first NUI 30 to slightly more than 0.5 milliseconds for data transmission during the second NUI 31, the transmissions from the nodes occur in a scheduled predetermined order, avoiding conflicts and arbitration problems between the data from the nodes.
Existing networks are considered conservative in data collection since data collection starts substantially immediately following the end of data transmission thereby leaving a maximum time for data collection prior the next tone 32. This conservative approach is beneficial in that it avoids having insufficient time to form data transmission packets needed for data transmission. However, a drawback which exists due to this early data acquisition is the possibility that data from a location which has already been read may be updated with more recent information. In this case, older than desired data may be sent during data transmission. Also, if data is missed and not sent during a first NUI, the time until the data is sent during a next NUI is extended, thereby causing data jitter.
It has been determined to be desirable to provide a timer in the scheduled controller based network to allow for issuing of a time variable data collection signal which indicates the beginning of data collection. The present invention, therefore, contemplates using a time variable data collection start signal to build a frame of scheduled transmission packets just before they are needed for transmission.